1. Field of the Invention
This invention relates generally to laser beam scanning devices and more particularly to a device employing one or more piezoelectric bimorphs for scanning a laser beam simultaneously in two orthogonal directions.
2. Description of the Prior Art
Piezoelectric materials are well known in the art and have found wide application in many fields. Ceramic piezoelectric materials such as lead zirconate titanate (PZT) are very rigid and are unsuitable for many applications. Piezoelectric polymers are less dense and more flexible than ceramic piezoelectric materials. One piezoelectric polymer that is an attractive alternative to ceramic piezoelectric materials in many applications is polyvinylidene fluoride (PVDF). PVDF has very different characteristics than its ceramic counterparts. For example, the charge coefficient of PVDF is about one-tenth that of PZT. Additionally, the alternating electric field strength that can be applied to PVDF without depolarization is about fifty times the field strength that can be applied to ceramic piezoelectric materials.
PVDF is typically manufactured as a film having a thickness in the range of 5 to 100 microns. FIGS. 1A and 1B graphically illustrate the well known properties of such films. A typical PVDF film 10 has electrode layers 12 deposited over its upper and lower surfaces for applying a voltage across the thickness of the film 10. During manufacturing, the film is stretched in a given direction as indicated in FIGS. 1A and 1B. A voltage applied across the thickness of the film induces an electric field across the thickness of the film 10. The polarization direction of the film and the direction of the electric film are indicated with the different arrows. As shown in FIG. 1(a), when the electric field has the same direction as the polarization direction of the film, the film will expand in the stretch direction. Conversely, as shown in FIG. 1(b), when the electric field and polarization directions are opposite, the electric field causes the film to shrink in the stretch direction.
Piezoelectric polymer films, and PVDF film in particular, are attractive materials for use in bimorphs. A bimorph is a structure consisting of two strips of piezoelectric film bonded together having their respective polarization directions arranged such that when an electric field is applied across both films, the field direction is the same as the polarization direction of one film, and opposite that of the other film. Consequently, one film will expand while the other shrinks causing the bimorph to bend in one direction. When an oscillating signal is applied across the films, the films alternately expand and contract causing the bimorph to oscillate. PVDF is well suited for bimorph applications due to its flexibility.
FIG. 2 illustrates the structure and operation of a typical piezoelectric bimorph 14. As shown, the bimorph 14 has a cantilevered end 16 fixedly attached to a support member 18, and a free end 20. The bimorph 14 comprises a first piezoelectric film 22 and a second piezoelectric film 24 bonded together by epoxy or any suitable material such that the respective electrode layers on the bonded sides of each film form a commonly connected electrode layer 26 between the films 22, 24. A voltage source 28 is coupled to electrode layers on the outside surfaces of each film 22, 24 and to the common electrode layer 26 for applying a voltage across the thickness of each film 22, 24. As described above, the films are bonded together with their respective polarization directions arranged such that, in one film, the electric field and polarization directions are the same, but in the other film, the electric field and polarization direction are opposite. An alternating voltage applied across both films will produce alternate expansions and contractions of the upper and lower films 22, 24 thereby causing the free end 26 of the bimorph 14 to vibrate or oscillate in a direction, A, substantially normal to the plane, P, in which the bimorph lies. The frequency of vibration is equal to the frequency of the applied oscillating voltage signal.
Recently, with the advent of optical scanning devices, such as bar code readers which are often found in retail stores, a need has arisen for small, inexpensive laser beam scanning devices capable of scanning a laser beam at relatively high frequencies. Bimorphs, such as that described above, are well suited for laser beam scanning devices. For example, a mirror (not shown) may be attached to the free end 26 of the bimorph 14. A laser beam incident upon the mirror will be scanned in the direction of vibration of the free end 26. Thus, a typical prior art cantilevered bimorph can be used to scan a laser beam in one direction. Bar code readers, however, require devices capable of scanning a laser beam simultaneously in two orthogonal directions because several different bar codes aligned in a parallel direction have to be read out serially. A cantilevered piezoelectric bimorph such as that illustrated in FIG. 2 is inadequate for such purposes.
Several devices exist in the prior art for scanning laser beams. For example, U.S. Pat. No. 3,758,199 issued to Thaxter discloses a laser beam scanning device comprising a pair of piezoelectric film transducers rigidly cantilevered at one end to a support member, but hingedly attached at the other end to the back of a mirror in spaced apart relation to enable pivotal motion of the mirror relative to the transducers. The device of Thaxter is to be distinguished from a bimorph in that the piezoelectric films do not bend, but rather the extension of one film and contraction of the other causes the hinged mirror to rotate about an axis passing through the mirror plane. Although the device of Thaxter is suitable for one-dimensional laser beam scanning, it is not capable of two-dimensional scanning.
U.S. Pat. No. 4,778,233 issued to Christenson et al. discloses a laser beam scanner comprising a pivotally mounted extended member having a reflective surface mounted thereon. A piezoelectric crystal abuts the extended member such that crystal deformation due to an applied voltage will cause the extended member to pivot. As with the device of the Thaxter patent, however, the device of Christenson et al. is not capable of two-dimensional scanning.
U.S. Pat. Nos. 4,775,815 and 4,917,484 issued to Heinz disclose a dynamic mount and actuator for a high energy laser beam steering mirror. The device of Heinz is designed for very precise mirror movements on the order of milliradians to be used in aircraft or space satellites. The complex device comprises three piezoelectric shear motors mounted 120.degree. apart on a cylindrical mounter mount. While two-dimensional laser beam scanning is possible with Heinz's device, it was not designed for applications such as bar code reading, and is much too complex and bulky to be suitable for such applications.
U.S. Pat. No. 4,251,798 issued to Swartz et al describes a two-dimensional laser beam scanning head for use in bar code reading systems employing a rotating polygonally shaped wheel having mirrors on each facet of the wheel to perform x-axis scanning of a laser beam. A single bimorph is used to achieve y-axis scanning. Alternatively, Swartz et al mention that a bimorph could be used for both x- and y-axis scanning, or that a single bimorph element of proper mechanical design electrically driven on both axes could be used; however, Swartz et al nowhere describe how to design such a two bimorph or single bimorph embodiment. Additionally, Swartz et al. do not teach the use of polymer piezoelectric films such as PVDF.
In U.S. Pat. Nos. 4,387,297 and 4,496,831, Swartz et al. disclose a penta-bimorph scanning element (FIG. 15). Swartz et al. mention that one penta-bimorph scanning element can be used for x-axis scanning and another for y-axis scanning. The penta-bimorph element of Swartz comprises a pair of oscillating elements which reciprocally oscillate in response to applied voltages. The bimorphs are each mounted at one end to a support structure, and mirrors are mounted on their upper ends and positioned so as to form a 45.degree. angle. The bimorphs oscillate in the same plane in a manner resembling a tuning fork. Two bimorphs are used in order to increase the scanning angle in a single direction because, individually, the bimorphs used by Swartz et al do not deflect at a large enough angle. Thus, the second mirror deflects the beam in the same direction as the first mirror and merely serves to increase the scan angle. A single penta-bimorph of Swartz et al. cannot provide two-dimensional scanning; rather, as Swartz et al. mention, two pentabimorphs must be used if two-dimensional scanning is to be achieved. Thus, although two-dimensional scanning is possible, two penta-bimorphs must be used thereby increasing the cost, complexity and size of a complete two-dimensional scanning device.
There is a need, therefore, for a laser beam scanning device capable of scanning a laser beam simultaneously in two orthogonal directions which is less complex than the prior art and takes advantage of the properties of piezoelectric films in general, and PVDF in particular. The present invention satisfies these needs.